It's hard to imagine there are enough LibreSignal users to make a notable impact on the OWS infrastructure in terms of expenses. Though over time, that could change, especially if other forks joined the party, and of course the fallout of shutting them out would only grow over time.
Given OWS's worries about federation and shared standards standing in the way of progress, they might want to avoid third party access simply because it might stop them from changing the server interface. Thirds party clients would be at the mercy of their developers when OWS makes an incompatible change. Caveat emptor, I would say, but again, the problem would only grow.
My personal situation is that the only people I talk to via Signal are nerds; the kind of people who jump ship when a sexier alternative comes up -- like we did when TextSecure/Signal offered good e2e crypto. But that's stopped being a unique feature since WhatsApp also uses the Signal Protocol, which simultaneously made it feel much less pressing to convince non-nerds to switch from WA to Signal.